Add a get_terminal_size function

[rchog]

I didn't get a reply to my issue/feature request, but saw that it was added to the Riot roadmap, so maybe it is a feature you'd want in this library. IANAP, but this was functionality I missed so I had a crack at it anyway. It's not pretty, but I was able to get the terminal dimensions in every terminal emulator I have installed, and with ~legacy:true it works in the Linux console as well.

Blocking for 0.01s seems like a lot, but xfce4-terminal (and presumably other VTE-based terminal emulators?) seems to need that much time to respond, at least on my machine.

I'm sure there's things I've overlooked, over-complicated, naming or style conventions I've broken etc. so feel free to treat this as a rough sketch and/or spam as you like, but any feedback would be welcome.